Hi Chirag --
A byte-swap operator would be a reasonable addition to the DWARF
interpreter.
In addition to writing a proposal as Paul suggests, one option I would
recommend you consider is to create a user-defined operation to perform
this byte-swap operation and give it a value in the range between
DW_OP_lo_user and DW_OP_hi_user. Of course, this user-defined would
need to be implemented not only in the producer (i.e., compiler) but in
any consumers (i.e., debugger).
When a new version of the DWARF Standard is released, this operation
would be assigned a new value.
On 10/28/19 6:53 AM, Robinson, Paul via Dwarf-Discuss wrote:
Hello Chirag,
Regarding a byte-swap operation, it seems that you have a reasonable
use-case on a bi-endian machine. Feel free to request a new operator on
the “public comments” page at http://dwarfstd.org/Comment.php
Note that a byte-swap operator would swap all bytes in the top-of-stack
value, which on your 64-bit machine would of course be a 64-bit value.
As you want a 32-bit swapped value, you would still need to do a shift
afterward, but even so, “DW_OP_byte_swap DW_OP_const1u 32 DW_OP_shr”
would be considerably shorter than what you have to do now.
Of course a new operator would be introduced in a new DWARF revision,
which is likely to be years away. In the meantime let me suggest a
shorter expression for doing the byte-swap operation. The book
“Hacker’s Delight” shows a straightforward 32-bit byte swap with masks
no wider than 16 bits, as follows:
x = (x << 24) | ((x & 0xff00) << 8) | ((x >> 8) &
0xff00) | (x >> 24);
Your 64-bit machine will of course use 64-bit values on the expression
stack, so to keep the result “32-bit clean” we want to do one additional
mask:
x = ((x & 0xff) << 24) | ((x & 0xff00) << 8) | ((x >> 8)
& 0xff00) | (x >> 24);
Translating this into a DWARF expression, I get the following:
DW_OP_dup, DW_OP_const1u 0xff, DW_OP_and, DW_OP_lit24,
DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_const2u 0xff00, DW_OP_and,
DW_OP_lit8, DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_lit8, DW_OP_shr,
DW_OP_const2u 0xff00, DW_OP_and, DW_OP_swap, DW_OP_lit24, DW_OP_shr,
DW_OP_or, DW_OP_or, DW_OP_or
I hope this is helpful to you.
--paulr
*From:* Dwarf-Discuss <dwarf-discuss-boun...@lists.dwarfstd.org> *On
Behalf Of *Chirag Patel via Dwarf-Discuss
*Sent:* Monday, October 28, 2019 12:47 AM
*To:* dwarf-discuss@lists.dwarfstd.org
*Subject:* [Dwarf-Discuss] dwarf stack operator for byte swap.
Hello Dwarf experts.
I am currently working trying to encode dwarf of binaries with having
bi-endian variables marked with DW_AT_endianity attribute.
The location calculation for some variable depends on other variable
with different endianity, also the value of this other variable is known
at runtime.
At the moment I am using location list to calculate the correct location
of first variable and list of dwarf operators to reverse the endianity
of variable “__gbloffset__” in below case (I only needed lower 32 bits
on 64 bit machine).
0x000001e5: DW_TAG_base_type
DW_AT_byte_size (0x04)
DW_AT_encoding (DW_ATE_signed)
DW_AT_name ("int")
DW_AT_endianity (DW_END_big)
…
0x00000057: DW_TAG_variable
DW_AT_name ("__gbloffset__")
DW_AT_type (0x000001e5 "int")
DW_AT_external (true)
DW_AT_decl_file ("…")
DW_AT_decl_line (8)
DW_AT_location (DW_OP_addr 0) // pre linkage
DW_AT_linkage_name ("_gblsection__")
0x00000170: DW_TAG_variable
DW_AT_name ("VAR1")
DW_AT_type (0x0000010b "fixed.dec.display.72")
DW_AT_decl_file ("…")
DW_AT_decl_line (10)
DW_AT_location (DW_OP_addr 0x0, DW_OP_call4 0x57,
DW_OP_deref_size, 4,
DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit0, DW_OP_shl, DW_OP_and,
DW_OP_lit24, DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_constu 0xff,
DW_OP_lit8, DW_OP_shl, DW_OP_and, DW_OP_lit8, DW_OP_shl, DW_OP_swap,
DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit16, DW_OP_shl, DW_OP_and,
DW_OP_lit8, DW_OP_shr, DW_OP_swap, DW_OP_constu 0xff, DW_OP_lit24,
DW_OP_shl, DW_OP_and, DW_OP_lit24, DW_OP_shr, DW_OP_swap, DW_OP_or,
DW_OP_or, DW_OP_or, DW_OP_plus)
DW_AT_linkage_name ("VAR1")
In above snippet of dwarf dump, I am using yellow highlighted list of
operators to swap the bytes.
I think there should be a support for DW_OP_byte_swap simple operator to
accomplice this simple task. Does this idea looks like it can be useful?
Is there any specific reason why dwarf spec does not have it or I am
missing something subtle.
I hope I conveyed the idea properly, apologies in advanced as English is
not my first language.
**
*Thanks and regards,*
**
*Chirag Patel*
Software Engineer | Raincode Labs India
*Tel*: (+91) 080 41159811
*Mob*: (+91) 9049336744
www.raincodelabs.com <http://www.raincodelabs.com/>__
linkedin-button <https://in.linkedin.com/in/chirag-patel->
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